Thigh muscle co-contraction patterns in individuals with anterior cruciate ligament reconstruction, athletes and controls during a novel double-hop test

Efficient neuromuscular coordination of the thigh muscles is crucial in maintaining dynamic knee stability and thus reducing anterior cruciate ligament (ACL) injury/re-injury risk. This cross-sectional study measured electromyographic (EMG) thigh muscle co-contraction patterns during a novel one-leg double-hop test among individuals with ACL reconstruction (ACLR; n = 34), elite athletes (n = 22) and controls (n = 24). Participants performed a forward hop followed by a 45° unanticipated diagonal hop either in a medial (UMDH) or lateral direction (ULDH). Medial and lateral quadriceps and hamstrings EMG were recorded for one leg (injured/non-dominant). Quadriceps-to-Hamstring (Q:H) ratio, lateral and medial Q:H co-contraction indices (CCIs), and medial-to-lateral Q:H co-contraction ratio (CCR; a ratio of CCIs) were calculated for three phases (100 ms prior to landing, initial contact [IC] and deceleration phases) of landing. We found greater activity of the quadriceps than the hamstrings during the IC and deceleration phases of UMDH/ULDH across groups. However, higher co-contraction of medial rather than lateral thigh muscles during the deceleration phase of landing was found; if such co-contraction patterns cause knee adduction, a putative mechanism to decrease ACL injury risk, during the deceleration phase of landing across groups warrants further investigation.

www.nature.com/scientificreports/ and actively playing in the highest or second highest league in their respective sport, and (3.) 24 asymptomatic non-athletic controls (20 females) who were not performing any specific knee-demanding activities, resistance exercises or participating in group training activities were considered acceptable. All groups were recruited through convenience sampling via adverts posted around the university campus, emails, and word of mouth. Individuals with ACLR were recruited, regardless of their physical activity level, if they were at the end phase of rehabilitation or had completed rehabilitation and were able to perform hops on the ACLR leg without any pain or discomfort. Participants with any history of hip, knee or ankle injuries within the past 6 months (other than ACL injury for ACLR participants) or diagnosed with any ongoing musculoskeletal, rheumatic, or neurological diseases were excluded. Each participant was examined by an experienced physiotherapist prior to the test sessions to ensure their eligibility. Self-reported knee function and physical activity levels of all participants were measured using the following questionnaires: Lysholm questionnaire, Knee injury and Osteoarthritis Outcome Score (KOOS), Tegner score, International Physical Activity Questionnaire (IPAQ)-short form, and the International Knee Documentation Committee (IKDC) form. In addition, quadriceps and hamstring isometric strength was assessed using the Kin-Com® dynamometer (Kinetic communicator 125 Auto Positioning, Chattanooga Group Inc.; Hixon, TN, USA).

Instrumentation. A Noraxon TeleMyo™ Direct Transmission System (DTS) Belt
Receiver (Noraxon Inc., USA), four DTS EMG transmitters, adhesive disposable silver/silver chloride surface electrodes, a 3-dimensional motion analysis system (Oqus, Qualisys, Sweden) with eight high-speed cameras, and two floor-embedded force plates (Kistler Winterthur, Switzerland; sampling-frequency: 1680 Hz) were used. The details of marker set configuration (56 retroreflective markers) and motion analysis are elaborated in our previous reliability studies on the one-leg double-hop test 31,32 . Procedure. Relevant demographic and anthropometric data were collected before testing. Each participant's self-preferred leg to kick a ball was noted as their dominant leg. All the participants performed the tests barefoot and wore a sports bra and/or tight training shorts.

Electromyography. The recommendations from the Surface Electromyography for the Non-Invasive
Assessment of Muscles (SENIAM) committee were followed for recording muscle activity 51 . Participants' skin over the recording site was shaved, abraded with sandpaper and swabbed with antiseptic wipes (75% isopropyl alcohol). Then Ag/AgCL surface electrodes were placed at an inter-electrode distance of 2 cm on the biceps femoris, medial hamstring, vastus lateralis and vastus medialis (Table 1) of one leg (ACLR group: the injured leg; control/athlete group: non-dominant leg). We chose the nondominant legs of healthy-knee athletes and controls to provide a more stringent comparison with the injured legs of ACL groups [52][53][54][55] . EMG data were recorded at 1500 Hz with Noraxon Telemyo 2400t G2 telemetric system (Noraxon Inc., Scottsdale, AZ, USA) using QTM software (Qualisys, Inc.).
One-leg double-hop test. The procedure was verbally explained to the participants at the start of the test session. Our one-leg double-hop test comprised of a forward hop followed by an immediate diagonal hop 45° to the medial or lateral direction in an unanticipated manner (see Fig. 1 and below for a more detailed description) 31,32 . Following two practice trials for each leg, participants were required to perform a minimum of 12 successful unanticipated diagonal hops (three per direction [medial/lateral] per leg). Inclusion of three trials for functional tests has been recommended by the IKDC 56 .
Participants stood on one foot on a force plate with their hands holding a 20 cm long rope (with knots at both ends) behind their lower back. They hopped forward onto the second force plate, and then performed a cutting maneuver (diagonal hop) in either the medial or the lateral direction at an angle of 45°5 7 (Fig. 1) over a predetermined distance (25% of their body height) guided by the light signal and further indicated by an adhesive tape affixed on the floor.
As for unpredictability of the hop direction, a projector mounted in the ceiling provided the visual cue (illumination of rectangles on the floor at a distance of 25% of their body height) upon initiation of the hop, to indicate whether the participant should attempt to land in a position medial or lateral to the direction of the forward hop. The order of direction was pseudorandomized. The visual cue was triggered as soon as vertical ground reaction Table 1. Surface electromyography-electrode placement guidelines. *Guidelines retrieved from SENIAM (www. seniam. org).

Muscle
Electrode placement* Electrode alignment* 50% on the line between the ischial tuberosity and the lateral epicondyle of  tibia   In the direction of the line between the ischial tuberosity and the lateral  epicondyle of the tibia   Medial hamstring  50% on the line between the ischial tuberosity and the medial epicondyle of  tibia   In the direction of the line between the ischial tuberosity and the medial  epicondyle of the tibia   Vastus lateralis  2/3 on the line from the anterior superior iliac spine to the lateral side of the  patella  In the direction of the muscle fibers   Vastus medialis  80% on the line between the anterior superior iliac spine and  www.nature.com/scientificreports/ force (VGRF) on the first force plate fell below 80% of its peak value during the push-off phase of the forward hop. Participants were required to perform the cutting maneuvers as quick as possible after receiving the visual cue indicating the hop direction. While hopping with the right leg, cutting done to the right side was named as ULDH and the left side as UMDH.

Biceps femoris
The trials were declared successful by an assessor during testing and then verified afterwards by another assessor using video analysis and Qualisys data. A ULDH or UMDH was considered successful if the participants hopped in the appropriate direction, covered the required distance and maintained balance and control upon landing without touching down their other foot. The trials were declared unsuccessful if the participants hopped in the opposite direction (performed UMDH instead of ULDH or vice versa), paused 58 , touched the force plate with the contralateral foot upon landing, landed outside the target (illuminated) areas, had extra hops upon landing or hands let go of the rope. Participants were allowed repeat the test till a minimum of three successful trails was achieved.
Two events based on vertical ground reaction force (VGRF) on the second force plate were used to define the stance phase of landing and for extraction of EMG data during the timeframe of interest: IC, defined by an increase in VGRF by 20 N for the first time, to toe-off, marked by a fall in the VGRF below 20 N. EMG and force data were synchronized through a square wave transmitted by the Qualisys system. Data processing. EMG data were band-pass filtered between 20 and 500 Hz through a fourth order Butterworth filter and then Root Mean Square (RMS) filtered with a 20 ms sliding window 59 to generate a linear envelope. Thigh muscles EMG activity of the hopping leg on the second force plate (Fig. 1) was averaged for the following phases: 100 ms prior to initial foot contact 60 (pre-landing phase), first 50 ms of foot contact 61 (IC), and IC to peak knee flexion 62 (deceleration phase) of the land-and-cut maneuver.
The peak EMG value observed during landing 46 , i.e., between the pre-landing and the end of the land-andcut maneuver was used to normalize linear envelopes at each phase of interest. An average of three successful trials for each participant (with the shortest stance time on the second force plate) was included in the analysis.
Mean EMG activity of the vastus lateralis and vastus medialis was divided by that of the medial hamstrings and biceps femoris muscles to calculate Q:H ratio. In addition, muscle CCI was defined as the concurrent activation of two muscle groups: EMG S /EMG L *(EMG S + EMG L ) 63 where EMG S is the EMG magnitude of the less active muscle and EMG L is the EMG magnitude of the more active muscle. This equation was applied for each data Figure 1. A novel unanticipated double hop performed with the right leg: 1. right foot planted on the 1st force plate; 2. forward hop landing of the right leg on the 2nd force plate at a distance of 25% of their height by reacting to a light signal indicating the target areas (rectangle boxes) to land and the subsequent direction of hopping (in a random order); 3. diagonal hop landing of the right leg in the lateral direction at an estimated angle of 45°and at a distance of 25% of their height. Images were captured from Visual 3D software (v5.02. 30 www.nature.com/scientificreports/ sample and the resulting curve was integrated for the (three) phases of interest 63 . A high CCI can be interpreted as a high level of muscle activity in both muscles while low CCI would imply either low activity in both muscles or that one of the muscles has high activity and the other has a low activity 63 . CCIs were calculated for the three landing phases of interest for the following muscle pairs: vastus lateralis and biceps femoris (lateral Q:H CCI), and vastus medialis and medial hamstrings (medial Q:H CCI). To determine whether CCIs were balanced between the medial and lateral sides, the medial-to-lateral Q:H co-contraction ratio (CCR) was calculated by dividing the medial Q∶H CCI by the lateral Q∶H CCI 42 .
Statistical analysis. Data were tested for normal distribution using the Shapiro-Wilks tests. Skewed data were log-transformed and subjected to parametric analysis given that the log-transformed data followed normal distribution. Multiple 3 (groups) × 3 (phases) mixed analysis of variance models (ANOVAs) were used with group as a between-subject factor and phase of landing as a within-subjects factor for each hop direction (UMDH/ULDH). Kruskal-Wallis tests were used to test between-group mean differences in participant characteristics as those data were skewed, except for normally distributed thigh muscle strength values subjected to one-way ANOVA. Post hoc multiple tests were adjusted with a Bonferroni correction. The level of significance was set at p < 0.05. For all statistical analyses, the Statistical Package for the Social Sciences (Version-27, IBM SPSS Statistics, USA) was used.

Results
Demographic and anthropometric characteristics of all participants and ACL injury-related data are summarized in Table 2. There was a significant difference in age between the ACLR and athlete groups (p = 0.020) where athletes were slightly younger than those with ACLR. Also, individuals with ACLR had greater knee laxity (p < 0.001) than controls and athletes when measured with KT1000 arthrometer (Table 2). Isometric peak torque of the quadriceps and hamstrings were not significantly different between the groups. Self-estimated knee function was lower for individuals with ACLR compared to controls and athletes ( Table 3). Level of physical activity also differed between groups where athletes were more active than controls and those with ACLR (p ≤ 0.039) as estimated by IPAQ. Individuals with ACLR had a median Tegner score of Table 2. Participant characteristics. Significant values are in [bold]. A, limb with anterior cruciate ligament reconstruction; ACLR, anterior cruciate ligament reconstruction; ND, non-dominant limb of the controls and athletes. *Post-hoc comparisons (p vlaues) abjusted by Bonferroni correction for multiple tests. **p value based on chi-square test. ***p values based on one-way ANOVA as these data were normally distributed; missing data for one athlete were omitted from analysis. ۴ KT1000 arthrometer device was used to measure knee laxity at 15, 20 and 30 lb force; data represent differences between ACLR and noninjured knees (ACLR group) or dominant and non-dominant side knees (controls and athletes); missing data for one with ACLR, two controls and one athlete were omitted from analysis. ^Missing isometric strength data for one with ACLR and one athlete were omitted from analysis. # Data missing for one participant with ACLR. @ Median (range). $ Mechanism of injury: 10 contact, 24 noncontact; Sport related to injury: 12 soccer, 10 floorball, 2 downhill skiing, 2 gymnastics, 2 rugby, 2 handball, 1 aerobic training, 1 mixed martial arts, 1 Kung-Fu.  www.nature.com/scientificreports/ 6 (range: 3-9) at the time of our study but their median pre-injury score was 9 (range: 3-10). All three groups differed significantly in activity level (athletes > ACLR > controls) according to the Tegner scores at the time of data collection. All but one person with ACLR were injured during sports participation and 23/34 (68%) had injured their dominant leg (Table 3) which was the right leg in 87% (20/23) of the cases.
Q:H ratio. The IC phase rendered a higher Q-H ratio than the pre-landing and deceleration phases, regardless of group, for both directions (Table 4) owing to a significant main effect for phases (p < 0.001) for Q:H ratio (Table 5). There was no significant interaction between groups and landing phases for UMDH or ULDH.

Medial and lateral Q:H CCIs.
For medial and lateral Q:H CCIs (UMDH and ULDH), all groups demonstrated a low CCI in the pre-landing phase with gradually increasing CCIs for the IC and deceleration phases. However, there was a significant interaction between groups and phases for medial and lateral Q:H CCIs for UMDH (p ≤ 0.030); however, post hoc tests did not reveal significant differences between groups for any of the phases. Even so, there was an increase in the mean (normalized EMG) scores of the groups through the phases, more so for individuals with ACLR and athletes. The deceleration phase had the highest Q:H CCI value amongst the phases of interest (Table 4). No interaction between groups and phases was evident for ULDH (p > 0.050; Table 5).

Medial-to-lateral Q:H CCR .
A significant main effect of phases (but not of groups) was found for UMDH (p < 0.001) which implied that the landing phases were different regardless of the group. The IC and deceleration phases had a higher value (muscle co-contraction: medial > lateral) compared to the pre-landing phase for ACLR and control groups (Table 4). Although main effects of phases were significant for ULDH (p = 0.019), post hoc comparisons did not reveal significant differences between phases (p > 0.050). Nevertheless, a trend similar to UMDH was observed for the phases of land-and-cut maneuver. No significant interaction between groups was found for the medial-to-lateral Q:H CCRs of UMDH and ULDH. Similar to Q:H ratios, the medial-to-lateral Q:H CCRs demonstrated a low ratio in the pre-landing phase and a high ratio in the IC and/or deceleration phases. In general, a higher medial thigh muscles activity and a relatively lower lateral thigh muscles activity were observed during the deceleration phase of landing. Medial thigh muscles activity was higher than that of the lateral thigh muscles in the IC phase compared to the other phases ( Table 4). Regardless of group, the pre-landing phase CCRs were lesser than those of the IC and/or deceleration phases for UMDH and ULDH. However, controls took longer (UMDH: 0.64 ± 0.17 s; ULDH: 0.67 ± 0.17 s) to perform the land-and-cut maneuver compared to those with ACLR (UMDH: 0.56 ± 0.14 s; ULDH: 0.62 ± 0.16 s) and athletes (UMDH: 0.49 ± 0.15 s; ULDH: 0.52 ± 0.15 s).
Box plots of Q:H ratios and medial-to-lateral Q:H CCRs for all three groups have been included as supplementary information. Table 3. Self-reported knee function and physical activity level presented as median (range: min-max) for participants with ACLR, control and athlete groups. Significant values are in [bold]. ACLR, anterior cruciate ligament reconstruction; IKDC, International knee documentation committee; IPAQ, International Physical Activity Questionnaire; KOOS, Knee injury and Osteoarthritis Outcome Score; TSK: Tampa Scale for Kinesiophobia. For all data in this table, data were missing for 2 participants (1 individual with ACLR and 1 athlete). **Post-hoc comparisons (p vlaues) abjusted by Bonferroni correction for multiple tests. † An activity scale ranging from 1 to 10 indicating current level of knee-demanding activities. ‡ A scale of estimated metabolic equivalent (MET-minutes/week) from total duration of physical activities throughout the previous week with a high score indicating greater physical activity level. *A scale ranging from 0 (worst) to 100 (best) indicating self-estimated knee function. § A scale of fear of movement with scores ranging from 17 to 68 with a high score indicating more fear of movement.

Discussion
Our study aimed to describe and compare the EMG activity of the quadriceps and hamstrings muscles during a novel one-leg double-hop test involving a forward hop immediately followed by a diagonal hop performed in medial or lateral direction between ACLR, athlete and control groups. The task, being unanticipated in nature, was designed to mimic sports-specific land-and-cut maneuvers, in which non-contact ACL injuries frequently occur. The Q:H ratio of all groups exhibited a similar trend for UMDH and ULDH with a particularly high quadriceps activity and a comparatively low hamstring activity in the IC and deceleration phases. On the other hand, hamstrings activity was more dominant than that of the quadriceps in the pre-landing phase (Table 4). A high hamstring activity prior to landing would cause eccentric deceleration of the tibia and prepare the knee for landing with optimal absorption of the forces created by ground contact. The high quadriceps activity with a Table 4. Median (interquartile range) values of thigh muscle co-contraction indices and ratios for UMDH and ULDH. ACLR, anterior cruciate ligament reconstruction; CCR, cocontraction ratio; Q:H, quadriceps to hamstring; Q:H CCI, quadriceps to hamstring cocontraction index; ULDH, unanticipated lateral diagonal hop; UMDH, unanticipated medial diagonal hop. Landing phases: pre-100 ms = pre-landing phase, 100 ms prior to initial foot contact; IC = 50 ms after initial foot contact; deceleration phase = IC to peak knee flexion.  therefore, degrees of freedom were adjusted with a Huynh-Feldt correction. *Significant interactions followed by post hoc tests with a Bonferroni correction did not reveal significant differences between groups for any of the phases; however, there was an increase in the mean (normalized EMG) scores of the groups over time (phase 1-phase 3), more so for patients with ACLR and healthy athletes. All analyses were performed with log transformed data. www.nature.com/scientificreports/ relatively low hamstring activity immediately after IC could be seen as a risk factor for ACL injury as it increases the anterior translation of the tibia over the femur 18,20,64 . However, a high sagittal plane load alone may not be enough to cause an ACL injury since the risk factors for the injury are multifactorial 20 , also involving other intrinsic and extrinsic factors. Discordant with our findings, a study by Ford et al. (2011) reported greater quadriceps (rectus femoris, vasti medialis and lateralis) activity compared with the hamstrings (biceps femoris and semitendinosus) in the pre-landing phase for healthy athletes while performing a bilateral drop-vertical jump from a height of 45 cm 65 . Conversely, in agreement with our findings, they did find hamstrings activity to be greater than the quadriceps in the pre-landing phase for drop-vertical jumps performed at lower drop heights (15 and 30 cm). Increased thigh muscle (quadriceps or hamstrings) activity in the pre-landing phase implies preparation for landing 66 owing to feed-forward control 67 associated with anticipation of variations in joint movements and forces required for task-specific landing 68 . However, the preferential activation of quadriceps with landing from an increased drop height might be due to increased demand of the task on the knee joint 65 . EMG activity patterns seem to be individual-and task-specific with different recruitment strategies. A recent systematic review did not find significant differences in quadriceps and hamstrings activity onset prior to one-leg landing or decelerating tasks between individuals with ACL injury/ACLR and asymptomatic controls 41 . There might be a lack of difference in EMG onset between thigh muscles during certain tasks which involve deceleration 41 which warrants further investigation in relation to UMDH/ULDH. However, in the current study, we did not investigate EMG onsets, and the magnitude of activity differed between thigh muscles prior to and during the land-and-cut maneuver of UMDH and ULDH.
For UMDH and ULDH, the medial and lateral Q:H CCIs gradually increased from the pre-landing phase to the deceleration phase (Table 4). This reveals a high contrast for quadriceps and hamstrings activity (vastus medialis vs. medial hamstring and vastus lateralis vs. biceps femoris) in the pre-landing phase that was gradually reduced over the IC and deceleration phases. This imbalance in CCIs occurred because of a greater increased recruitment of the hamstrings compared to the quadriceps for both medial and lateral muscle groups in the prelanding phase; however, vice versa was true for the IC and deceleration phases. Increased hamstring activity prior to landing reflects preparation for landing 66 which could be mediated by feed-forward control 67 in anticipation of joint movements and loads associated with landing 68 . Even so, similar to our findings, quadriceps activity has been reported to be higher than the hamstrings during the deceleration phase of side-step cutting 69 and jump landing maneuvers 70 . Increased quadriceps activity could increase anterior tibial translation and strain the ACL from 0° to 45° of knee flexion 71 . However, clinicians and also individuals with ACLR should note that sagittal translation of the tibia remains lower in closed kinetic chain exercises compared to open kinetic chain exercises 72 incorporated in rehabilitation.
The medial-to-lateral Q:H CCRs were not significantly different between groups. The IC and deceleration phases had a higher value than the pre-landing phase for ACLR and control groups. The medial-to-lateral Q:H CCRs (Table 5) indicate predominant coactivation of the medial thigh muscles in relation to that of the lateral group. This might cause knee adduction and thus counteract external knee abduction moments 64 during the IC and deceleration phases. These findings seem to be concordant with the observed knee adduction angles during the deceleration phase (UMDH: ACLR, 7.36° ± 4.93°; athletes, 7.57° ± 6.39°; controls, 7.62° ± 4.43°; ULDH: ACLR, 6.85° ± 4.70°; athletes, 10.74° ± 7.32°; controls, 9.51° ± 6.64°; methods for kinematic analysis are reported elsewhere 31,32 ). However, we found weak insignificant Pearson correlations (r < 0.30; p > 0.05) between medialto-lateral Q:H CCR and knee adduction angles for the deceleration phase of the land-and-cut maneuver for all three groups for both (medial and lateral) direction of hops. Although these correlations addressed the frontal plane mechanics, it is important to consider the relationship between multiplanar movement combinations and CCIs/CCRs of the knee in subsequent studies. Whether medial-to-lateral Q:H CCR, medial Q:H CCI or medial quadriceps and hamstring EMG amplitudes would be associated with a significant variation in the frontal plane knee angles, and external or internal moments during different phases of landing warrant further detailed investigation in the future studies. We recommend incorporating various covariates (e.g., most importantly ACL injury status, comorbidities [e.g. osteoarthritis, concomitant ligament injuries, etc.], kinesiophobia, dynamic knee stability measures, physical activity level, self-reported outcome measures, knee laxity, time since surgery/ injury) while analyzing the relationship between multiplanar knee angles/moments and thigh muscle CCIs/CCRs or temporal parameters (EMG onsets and duration of co-contraction of muscle pairs).
Prior training might help with neuromotor planning and anticipatory contraction of the lower limb muscles that control and stabilize the knee 42 . Nevertheless, there is a lack of consensus regarding an optimal neuromuscular training to be implemented following ACLR 73 . Zebis et al. found that neuromuscular training aimed at preventing non-contact ACL injuries might help to increase selective activation of the medial hamstrings to decrease external rotation and abduction of the knee during instep and side-cutting maneuvers 47 . In the current study, we documented only the current physical activity levels of the healthy controls based on the IPAQ and Tegner scores; therefore, their prior experience with cutting maneuvers or knee demanding activities was not fully explored. It cannot be ruled out whether all three groups (ACLR, athletes and controls) may have experienced some form of training that resulted in a lack of difference between groups.
Some methodological considerations pinpointing limitations of the study are summarized below. Performing UMDH and ULDH in a controlled laboratory environment will not exactly emulate sports-specific cutting maneuvers endangering ACL integrity. Though our participants were asked to perform UMDH/ULDH as quickly as possible by reacting to the visual cue, this may still have taken a longer time than that to perform a cutting maneuver in a real sporting situation.
We assumed that a difference in males:females ratio (males ≤ 26% in each group) might not affect betweengroup comparisons owing to that the proportion was similar in all groups. Despite a statistically significant difference in age of participants (ACLR vs. athletes), they were not expected to have any age-related changes of www.nature.com/scientificreports/ physical attributes affecting thigh muscle activity. Participants (aged 17-34 years) in all groups were screened for musculoskeletal impairments by an experienced physiotherapist prior to data collection. As we have included participants with a hamstring graft in the ACLR group, whether it would adversely affect medial hamstring activity remains ambiguous. Following a semitendinosus graft the regeneration of the semitendinosus tendon after 1-3 years 74,75 could result in a more proximal insertion of the distal tendon (⁓4 cm) and a decreased muscle moment arm. This might lead to an increase in EMG activity of the medial hamstring because of an increased motor unit activation to produce more force to match an equal amount of muscle torque 76 . Individuals with ACLR had the surgery 7 to 129 months (median 18 months) prior to their participation in the study and the extent of regeneration of the semitendinosus tendon and associated changes in EMG co-contraction patterns might be variable among the participants (undergoing rehabilitation vs. returned to functional activities/sports). A longitudinal prospective cohort study thoroughly addressing the influence of time aspect on such outcomes is warranted.
A novel standardized rebound side-hop recently demonstrated that individuals with ACLR with a high fear of re-injury display significantly higher thigh muscle co-contraction when compared to ACLR individuals with a low fear of re-injury and asymptomatic controls 77 . Our knee-challenging one-leg double-hop test might be even more suitable to investigate objective measures of movement coordination in relation to fear of re-injury, which is one of the major reasons preventing return to sports 78 .
Future studies can also compare variations in our double-hop test and subsequent changes in EMG co-contraction patterns of individuals with and without ACL injuries; for instance, incorporating penultimate contact with the non-dominant foot followed by final foot contact with the dominant foot or vice versa at varying angles of direction change (45°, 90°, etc.) 79 . We recommend recording gastrocnemius (an antagonist of the ACL) 80 activity along with the thigh muscles and analyze the relationship of EMG to knee angles and moments in the future studies. Comparing unanticipated hops with pre-planned hops is further warranted.

Conclusion
An increased quadriceps activity compared to that of the hamstrings, accounting for a higher Q:H ratio, was found during the IC and deceleration phases compared to the pre-landing phase of the land-and-cut maneuver (UMDH/ULDH) for all groups. All groups, irrespective of ACL injury or physical activity level, showed a relatively low imbalance in the medial and lateral Q:H CCIs for the pre-landing phase compared to the IC and deceleration phases of landing. However, controls took a longer time to complete the task compared to individuals with ACLR and elite athletes. Overall, the findings indicate predominant co-contraction of the medial thigh muscles over the lateral group; if such co-contraction patterns cause knee adduction during the deceleration phases of the land-and-cut maneuver needs further substantiation. Our study results would be useful for clinicians and researchers investigating the role of thigh muscle activation patterns in augmenting or mitigating ACL injury risk during unanticipated one-leg landing tasks.

Data availability
Data are available at reasonable request.